Experiments are designed to evaluate theories concerning the functional significance of cerebrospinal fluid (CSF). Two experimental approaches are proposed: 1. The major difficulty in assessing the hypothesis that CSF functions as an excretory system for brain has been to determine the characteristics of material transport from brain to CSF. Proposed experiments are designed to investigate this problem by following the time course and route of removal from rat brain of test substances injected into identified intracerebral sites via chronically implanted cannulae. 2. Primitive characteristics of CSF and brain-barrier mechanisms in nurse shark, dogfish and hagfish recommend these species as experimental models for studying the functional significance of CSF. Proposed experiments aim to further characterize relevant anatomical and physiological aspects of CSF and brain-barrier mechanisms.